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Chemistry and Mineralogy of Ferromanganese Deposits from the Equatorial Pacific Ocean

1992; Springer Nature; Linguagem: Inglês

10.1007/978-1-4612-2896-7_12

1068-6517

Eric H. De Carlo, Charles M. Fraley,

Geology and Paleoclimatology Research

Ferromanganese nodules, crusts, and phosphatic substrates were recovered during two cruises of the R/V Moana Wave to the Cook Islands, Kiribati, and Tuvalu. We report the geochemistry and mineralogy of ferromanganese crusts and seamount nodules only. Samples were collected from flanks, gentle slopes, and nearly flat summits; nodules were absent from the steeper slopes. Cobalt, the metal of highest commercial interest, ranges between 0.19% and 1.90% by weight in deposits consisting of stains of less than one millimeter to a maximum of 12 cm thick. The highest Co dredge averages of 1.72% and 1.30% were found' in crusts recovered near Vaitupu and Enderbury islands respectively. Most crusts from the Manihiki Plateau were less than one centimeter thick and exhibited low metal values in contrast to those from a region of mud volcanoes on the plateau in which Co concentrations averaged 1.0%. Copper and Ni dredge averages ranged from 0.05% to 0.14% and from 0.19% to 0.89% respectively.The results of this study are generally in agreement with observations in other areas of the Pacific, and confirm the following criteria for seeking high metal grades in vernadite-rich ferromanganese deposits: 1) water depth of 1000–2000 m, 2) latitude 2–7°S, and 3) low sedimentation rates. Crusts of potential commercial interest occur primarily in the vicinity of Vaitupu and Enderbury Islands, but other gently sloped ridges and seamount summits in the Line Islands (near 1°S, 156°W) have abundant nodules or encrusted cobbles averaging 0.78% Co and 0.63% Ni.The mineralogy of crusts is dominated by vernadite with few samples enriched in todorokite. Seamount nodules are chemically and mineralogically distinct from their abyssal counterparts; like crusts they are primarily composed of vernadite. No crystalline iron phases were identified within Fe-Mn deposits. Phosphate bands are generally absent within crusts but commonly occur in substrates of samples collected above 2500 m. The phosphorite material in crusts and substrates is primarily carbonate fluorapatite replacing calcium carbonate.